1. Field of the Invention
This invention relates to a dyeing method for fibrous products using an oxidation-reduction dyeing type dye and more particularly to a dyeing method in which the reduction of the oxidation-reduction dyeing type dye is carried out using a new reducing bath.
2. Description of the Prior Art
In the dyeing of fibrous products using oxidation-reduction dyeing type dyes such as vat dyes and sulfur dyes, it is sodium hydrosulfite (hereinafter referred to simply as "hydrosulfite") that has heretofore been mainly used as a reducing agent. However, hydrosulfite is poor in preservative stability and undergoes an oxidative decomposition to a large extent upon contact with air, for which reason it is an actual situation that hydrosulfite is used in large excess amount as compared with a theoretically required amount. Such decomposition of hydrosulfite is specially remarkable in a continuous pad dyeing, in which the decomposition proceeds to a remarkable extent during a short period of time from the padding of a reducing solution on the cloth until steaming. Moreover, when handling, hydrosulfite produces a bad smelling gas; therefore, also in point of public nuisance based on a bad smell there has been a demand for a reducing agent free from smell.
Recently, thiourea dioxide has attracted attention and is being studied, as a reducing agent which has a good stability in an alkali solution, less undergoes an oxidative decomposition and is free from a bad smell.
Thiourea dioxide, which is also called aminoiminomethanesulfinic acid or formamidinesulfinic acid, is sold on the market industrially and is available as a white powder superior in preservative stability and having neither oxidizing property nor reducing property, but displays reducing property when an aqueous solution thereof is made alkaline or heated. Its reducing power is very large, stability in an aqueous alkali solution is good and decomposition by an air oxidation is less remarkable, with little production of a bad smell. Thus, thiourea dioxide is an excellent reducing agent. For this reason, application of thiourea dioxide to various fiber fields including dyeing, printing, discharge printing and reduction clearing has been studied and announced in literatures, etc. In the field of dyeing, various studies for practical application have been made, but some dyes undergo reduction to an excess degree, that is, the phenomenon of a so-called "over reduction" appears, one cause of which resides in the too large reducing power of thiourea dioxide. As a result, in the next oxidation process the dye changes into a substance which no longer returns to the original structure, thus causing abnormality in the hue, dye concentration, etc. of the resulting dyed product. This phenomenon is not observed in most dyes, but blue indanthrone dyes, which are widely used, undergo such phenomenon to a remarkable extent, and using hydrosulfite as a reducing agent causes a marked difference in hue and in dye concentration from the case of using thiourea dioxide as a reducing agent. This is an obstacle to the practical application of thiourea dioxide in the field of dyeing. Methods of preventing such abnormality in hue and in dye concentration have heretofore been proposed, including the use of additives to prevent the over reduction, and a combined use with other reducing agent.
So far announced as anti-over reduction agents which are also effective in the reduction with hydrosulfite are protective colloidal substances consisting of polysaccharide such as glucose, dextrin and mannose, as well as oxidizing agents such as sodium nitrite, sodium chlorate and nitro compounds, e.g. nitropropane and nitrobenzene. However, the effect of these substances differs according to the amount and concentration of thiourea dioxide, dyes, or the above additives and also the condition used. It is a present situation that even if certain concentration and condition afford a satisfactory result, a slight change in the concentration of dye, thiourea dioxide, etc. can give an unsatisfactory result not employable in practice.
Japanese patent publication No. 47066/1977 proposes a method in which thiourea dioxide is used together with hydrosulfite in an amount of 0.05 to 100 wt.% based on the hydrosulfite used. However, since the hydrosulfite is a principal component in this proposal, the drawbacks associated with hydrosulfite are not eliminated. In addition, for dyes which easily undergo over reduction, the amount of thiourea dioxide to be used is limited. Thus, such proposed method does not give a substantial solution.